Indian Monsoon Mechanism, Branches and Retreat

What the Monsoon Is, and What It Is Worth

"Monsoon" comes from the Arabic mausim, meaning season. Arab sailors used the word for the winds that blow from the south-west in summer and from the north-east in winter. The defining feature is a seasonal reversal of wind direction, which is why NDA 2023-II asked which climate type is marked by it (answer: the monsoon climate).

The India Meteorological Department (IMD) divides the year into four seasons:

The benchmark is the Long Period Average (LPA) of the June–September rainfall for the whole country. It is 87 cm, based on 1971–2020. IMD calls a season normal when rainfall is 96–104% of the LPA, deficient below 90% and excess above 110%. Roughly half of India's net sown area depends on rain, so these percentages decide the kharif harvest, rural incomes and food inflation.

Five Forces That Drive the South-West Monsoon

Textbooks once explained the monsoon as a giant sea breeze. That theory goes back to Edmund Halley in 1686: land heats faster than sea, so air rises over hot land and moist sea air flows in. That is still the starting point, but it cannot explain the timing, the sudden onset or the failures. The modern explanation adds four more forces.

1. Differential heating. By May, an intense low-pressure area forms over north-west India and Pakistan. A high-pressure area, the Mascarene High, sits over the southern Indian Ocean near Madagascar. Air flows from the high to the low.
2. The ITCZ moves north. The Inter-Tropical Convergence Zone, the belt where the trade winds of the two hemispheres meet, shifts north with the overhead sun. In July it lies at about 20–25°N over the Ganga plain, where it is called the monsoon trough. The south-east trades of the southern hemisphere cross the equator, and the Coriolis force bends them to the right, so they reach India as the south-west monsoon.
3. The Tibetan Plateau. In summer the plateau acts as a raised hot plate. The heated air rises and builds an upper-level high. That high helps form the Tropical Easterly Jet, which blows from east to west high over peninsular India in summer and strengthens the monsoon circulation.
4. The westerly jet jumps north. In winter the subtropical westerly jet flows south of the Himalayas. In early June it shifts north of the Tibetan Plateau. Its departure removes a lid on the atmosphere, and the monsoon arrives with a sudden "burst" of heavy rain.
5. Ocean signals far away.
   • An El Niño (warm waters in the eastern Pacific) usually weakens the Indian monsoon, and a La Niña usually strengthens it.
   • A positive Indian Ocean Dipole (warmer western Indian Ocean) tends to help.
   • The Madden–Julian Oscillation, a pulse of clouds moving east along the equator, causes the active and weak spells within a season.
   • Near the surface, the Somali (Findlater) jet, a fast low-level stream off the coast of East Africa, carries moisture across the Arabian Sea.

Onset, Burst, Break and Withdrawal

IMD revised its "normal dates" in 2020, using data from 1961–2019 for onset and 1971–2019 for withdrawal. These are the figures to quote:

Break in the monsoon. In some weeks of July and August, the monsoon trough shifts north to the foothills of the Himalayas. Rain then stops over the plains for a week or more, while the hills and the Brahmaputra valley flood. A long break during sowing is what turns a "normal" monsoon into a bad year for farmers.

Cloudburst. IMD defines a cloudburst as about 10 cm of rain in an hour over a small area of roughly 20–30 sq km. Cloudbursts are most common in the Himalaya during the monsoon, and they trigger flash floods and landslides (NDA 2022-I, below).

Two Branches, and Why the Rain Is So Uneven

The peninsula's shape splits the monsoon in two.

The Arabian Sea branch

• Western Ghats: the winds strike the Ghats almost at right angles. They rise, cool and pour more than 250 cm of rain on the windward slopes. Crossing the crest, the air descends and warms, leaving a rain-shadow on the Deccan plateau. That is why Pune and the interior of Karnataka are far drier than the coast only 100 km away.
• Narmada–Tapi trough: a second stream enters through this corridor into central India.
• Gujarat and Rajasthan: a third stream runs through Saurashtra and Kutch into Rajasthan. The Aravallis lie parallel to its path, so nothing forces the air to rise. Western Rajasthan stays the driest part of the plains.

The Bay of Bengal branch

• It strikes the coast of Myanmar and the Arakan Yoma, then turns into north-east India.
• In Meghalaya, the funnel-shaped valleys of the Garo and Khasi hills trap it. Mawsynram and Cherrapunji (Sohra) receive the heaviest rainfall in the world.
• The Himalaya then deflects it westward along the Ganga plain, and it grows drier as it goes, from Kolkata to Patna to Allahabad to Delhi.
• The two branches merge over the north-western Ganga plain.

Overall, rainfall exceeds 200 cm on the west coast and in the north-east. It falls below 50 cm in western Rajasthan, parts of Gujarat, the interior Deccan and Ladakh. The Köppen codes that capture this pattern, such as Amw on the west coast south of Goa, BWhw in the extreme west of Rajasthan, and As on the Coromandel coast (the 's' marks a dry summer, which is the north-east monsoon pattern), are explained on our Köppen classification page.

The Retreat and the North-East Monsoon

From mid-September the land cools, the low over north-west India weakens, and the monsoon trough slides south. The withdrawing monsoon leaves clear skies with high temperature and humidity behind it. This muggy spell is called "October heat". By early November the low-pressure area has moved over the Bay of Bengal. That is why October and November are the peak months for tropical cyclones that hit the east coast, as Cyclone Arnab did at Yanam this October. Our explainer on how Bay of Bengal storms are named and ranked covers the cyclone side.

The winds now blow from the north-east. Over most of India they come from the land, so they are dry. Before reaching the south-east coast, however, they cross the Bay of Bengal and pick up moisture. This north-east monsoon (October–December) waters:

• Tamil Nadu and Puducherry;
• coastal Andhra Pradesh and Rayalaseema;
• south interior Karnataka and Kerala.

The CDS asked exactly this in 2018: South Arcot and Ramanathapuram, two Coromandel districts, get more than half their rain from the north-east monsoon.

Western Disturbances and Pre-Monsoon Storms

Two smaller rain-makers fill the gaps.

Western disturbances

• These are low-pressure systems that originate over the Mediterranean Sea and western Asia. They are not born over the Caspian, a trap in NDA 2026-II.
• The westerly jet steers them into India in winter.
• They bring snow to Jammu and Kashmir, Ladakh and Himachal, and light rain to Punjab, Haryana and western Uttar Pradesh.
• That winter rain is very valuable for the rabi crop, above all wheat (NDA 2023-II).

Pre-monsoon storms (March–May)

• Nor'westers / Kalbaisakhi: violent evening thunderstorms in West Bengal, Odisha and Assam (where they are called Bardoli chheerha). They help the jute, rice and tea crops.
• Mango showers: pre-monsoon showers in Kerala and coastal Karnataka that help mangoes ripen.
• Blossom showers: rain that makes coffee flowers bloom in Kerala and the nearby hills.
• Loo: hot, dry, dusty westerly winds that blow in the afternoons over the northern plains in May and June.

The monsoon also leaves a record behind. Our article on a dead meander near Lucknow that stores 4,600 years of monsoon history shows how scientists read past rainfall in lake sediments.

How UPSC Has Asked It: Past Papers Solved

These are actual CDS and NDA questions since 2018. The monsoon is asked through places (which district, which state) and through mechanisms (which wind, which crop).

Practice MCQs (with answers)

1. The Long Period Average of India's south-west monsoon rainfall, based on 1971–2020, is about
   1. 78 cm
   2. 87 cm
   3. 98 cm
   4. 116 cm
2. The sudden onset of heavy rain at the start of the monsoon is linked to
   1. the westerly jet moving south of the Himalayas
   2. the arrival of western disturbances
   3. the subtropical westerly jet shifting north of the Tibetan Plateau
   4. a La Niña in the Atlantic
3. Western Rajasthan receives little rain from the Arabian Sea branch mainly because
   1. the Aravallis lie parallel to the wind direction
   2. it lies in the rain-shadow of the Western Ghats
   3. the Bay of Bengal branch blocks it
   4. the Loo blows all through the monsoon
4. Which of the following is the normal date by which the monsoon covers the entire country, as revised by IMD in 2020?
   1. 1 July
   2. 15 July
   3. 30 June
   4. 8 July
5. "October heat" refers to
   1. the hot dry Loo of the Ganga plain
   2. high temperature and humidity after the monsoon withdraws
   3. heat waves caused by El Niño
   4. temperatures before the onset over Kerala
6. Nor'westers are most closely associated with which crops?
   1. Wheat and barley
   2. Cotton and groundnut
   3. Jute, rice and tea
   4. Coffee and pepper
7. Which factor generally weakens the Indian south-west monsoon?
   1. An El Niño in the eastern Pacific
   2. A positive Indian Ocean Dipole
   3. A strong Tropical Easterly Jet
   4. A deep low over north-west India
8. The world's heaviest rainfall at Mawsynram is caused mainly by
   1. the Arabian Sea branch striking the Western Ghats
   2. western disturbances
   3. the north-east monsoon
   4. the Bay of Bengal branch trapped by the Garo and Khasi hills

Quick Revision